MIPS: ingenic: Add support for GCW Zero prototype

Add support for the GCW Zero prototype. The only (?) difference is that
it only has 256 MiB of RAM, compared to the 512 MiB of RAM of the retail
device.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>